This invention pertains generally to gas tungsten arc welding processes and more particularly to such processes applied to deep, narrow grooves.
Gas tungsten arc welding processes achieve coalescence by heating the weld zone with an arc struck between a tungsten electrode and the workpiece. To prevent oxidation, the heated weld zone, the molten metal and the non-consummable electrode which carries the welding current, are shielded from the atmosphere by an inert gas stream which is directed from the electrode holder, commonly referred to as the welding torch.
Most commercial torch designs employ a collet within the main torch body for supporting and transferring weld current to the tungsten electrode. The electrode generally extends from the collet, axially through a surrounding gas nozzle and projects a short distance beyond the nozzle orifice. The shield gas is conveyed through the nozzle, along the electrode and out the orifice to the weld. Such an electrode is described in copending Application Ser. No. 077,210, cited above.
Shield gas coverage is an important factor in establishing a successful weld with the gas tungsten arc welding process. Adequate shield gas coverage is critically important when welding high strength, low alloys steels, such as A533, which is used in the manufacture of steam generators. This material is extremely difficult to weld utilizing the gas tungsten arc welding process due to the high stresses set up in the material from the quench and temper techniques employed in its manufacture. The highly stressed material readily oxidizes on contact with air. A superheated welding puddle applied with the tungsten inert gas arc welding process on such carbon and low alloy steels shows a higher propensity for oxidation than would be encountered in welding higher alloy steels. High porosity, excess surface oxidation, embrittlement and cracking can result from a lack of adequate shielding in such applications. Because of these characteristics of carbon and low alloy steels and the difficulty in obtaining adequate shield gas coverage, the welding industry has experienced only limited success in applying the gas tungsten arc welding process. A more conventional slag producing weld process is usually employed through the industry for joining materials of this type in semiautomated manufacturing applications. While the conventional slag bearing welding process is satisfactory for a number of semiautomated welding operations, it has not produced as high quality of weld as the tungsten inert gas arc welding process. In addition, the slag bearing process requires that the welding operation be performed in a flat, downhand (1G) position which is not always practical. For example, in a number of manufacturing arrangements and field service applications it is desirable to apply the weld in a horizontal (2G) position.
The ability to achieve adequate shield gas coverage while utilizing the gas tungsten arc welding process is further complicated when the process is applied in a deep, narrow groove. A deep, narrow groove weld prep is desirable because it reduces the amount of weld deposit required and the time it takes to complete the weld. Such grooves may extend an excess of 3 inches in depth, have a root radius in the order of 7/32 inch radius or smaller and as little as a 1.degree., to a 6.degree. to 8.degree., included angle. The geometry and restrictiveness of the groove thus makes it difficult to maintain complete gas coverage of the weld puddle until coalescence is achieved. In addition, the use of a light shield gas, composed of, for example, 75% helium and 25% argon, which is desirably employed in such applications, adds to the difficulty in establishing good shield coverage. Furthermore, while welding in the 2G position, the hot metal adjacent the weld groove heats the surrounding air which rises and creates a draft in the weld groove (chimney effect) which draws away the gas coverage.
Accordingly, one object of this invention is to provide an improved gas shielded tungsten arc welding process that will provide adequate shield gas coverage in a deep narrow groove weld joint configuration. Another object of this invention is to provide such a process that will provide adequate shield gas coverage while welding in the 2G position. In addition, it is an object of this invention to provide such a welding process that will provide quality welds on high-strength, low alloy steels, such as A533.